Preventing Large Thermoelectric Voltages

This section includes several general techniques that prevent the appearance of large temperature gradients at critical components.

Reduced Heat Generation

When a Printed Circuit Board's (PCB) thermal gradient is mainly caused by the components attached to it, then you need to find components that dissipate less power. This can be easy to do (e.g., change resistors) or hard (change a PICmicro microcontroller).

Increasing the load resistance and other resistor values also reduce the dissipated power. Choose lower power supply voltages, where possible, to further reduce the dissipated power.

Redirect the Heat Flow

Changing the direction that heat flows on a PCB, or in its immediate environment, can significantly reduce temperature gradients. The goal is to create nearly constant temperatures in critical areas.

Alternate Heat Paths

Adding heat sinks to parts that dissipate a lot of power will redirect the heat to the surrounding air. One form of a heat sink that is often overlooked is either ground planes or power planes in the PCB; they have the advantage of making temperature gradients on a PCB lower because of their large (horizontal) thermal conductivity.

Adding a fan to the design will also redirect heat to the surrounding air, which reduces the temperature drop on the PCB. This approach, however, is usually avoided to minimize other design issues (random temperature fluctuations, acoustic noise, power, cost, etc.). It is important to minimize air (convection) currents near critical components. Enclose either the parts with significant temperature rise or the critical parts. A conformal coating may also help.

Isolation from Heat Generators

It is possible to thermally isolate critical areas on the PCB. Regions with little or no metal act like a good thermal insulator. Signals that need to cross these regions can be sent through series resistors, which will also act as poorly conducting thermal elements.

Place heat sources as far away from critical points as possible. Since many heat sources are in the external environment, it can be important to place these critical points far away from the edges of the PCB. Components that dissipate a lot of power should be kept far away from critical areas of the PCB.

Low profile components will have reduced exposure to the external environment. They may have the additional advantage of reduced electrical crosstalk.

Thermal barriers, such as conformal coating and PCB enclosures, can be helpful too. They usually do not have to be added unless there are other compelling reasons to do so.

Slow Temperature Changes

In some applications, sudden changes in thermoelectric voltages can also be a concern. Avoid power-up and power-down thermal transient problems by minimizing the currents drawn during these times. Also, reducing the times can help. Quick changes in voltages at heavy loads can be another source of concern. If the load cannot be made lighter, then isolation is usually the best approach to solving this problem.

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